Additionally or fully recordable optical recording disks of the type having recording and reflective layers on a substrate were proposed in accordance with the compact disk (generally known as CD) standard See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and SPIE, Vol. 1078, Optical Data Storage Topical Meeting, 80, 1989.
This type of disk has a dye layer, a reflective Au layer, and a protective layer disposed on a transparent resin substrate in this order. That is, the reflective layer is in close contact with the dye layer. As opposed to the prior art disk of the air sandwich structure wherein an air layer is provided on a dye layer of the disk in order to allow pits to be formed in the dye layer, the newly proposed disk is of the close contact type wherein the reflective layer is close to the dye layer. The close contact type configuration could meet the total disk thickness of 1.2 mm required by the CD standard.
In the medium of the close contact type wherein a reflective layer is close to a recording layer containing a dye, the recording layer should have a reflectivity of at least 60% with respect to the recording and reproducing light, especially at least 70% in the case of the CD standard. To this end, a gold thin film having high reflectivity and corrosion resistance is used as the reflective layer as disclosed in Japanese Patent Application Kokai No. 79235/1990.
However, the gold thin film has several problems. First, gold is soft. The gold thin film on the recording layer can be deformed by thermal expansion of the recording layer when pits are formed therein by directing recording light to the recording layer. This results in a disturbed eye pattern as reproduced output waveform and increased jitter.
Secondly, gold is expensive. It was proposed to use thin films of silver, copper and other less expensive metals as the reflective layer. For example, Japanese Patent Application Kokai Nos. 79235/1990 and 87341/1990 discloses thin films of Ag and Cu as the reflective layer. The Ag and Cu thin films provide a reflectivity of about 70% as deposited, which is gradually lost due to poor corrosion resistance. Acceptable reflectivity is not available with thin films of metals other than Ag and Cu, for example, aluminum thin films. Therefore, optical recording disks having thin films of metals other than Au as the reflective layer are increased in error rate and thus impractical.